Does every attorney need to build their own signature?
No. The firm sets the logo, layout and confidentiality footnote once, and each attorney's own name, title and headshot sit on top of it. Nobody retypes the shared parts.
Can the confidentiality footnote go on every attorney's signature automatically?
Yes. A compliance footnote the firm writes once shows under every member of the firm's signature automatically, without anyone copying it into their own signature by hand.
Is this legal advice on what a law firm's email must say?
No. Signatoro sets the layout and text you give it; what a firm's email actually needs to disclose is a compliance question, not a template question. See email signature compliance for the general rules that come up most often around a company signature.